1. Field of the Invention
The present invention relates to a torque transmission device.
2. Description of Related Art
The torque transmission device used in a joint of a propeller shaft provided in a vehicle is, for example, disclosed in Japanese Patent Application Publication No. 62-110628 (JP-U-62-110628). This torque transmission device includes, as shown in FIG. 6: a shaft 81 that has a male spline 81a in a part of an outer periphery; a sleeve 82 that has a female spline 82a fitted to the male spline 81a in a part of an inner periphery; a cylindrical cover 83 that is attached to a base portion 81b of the shaft 81 and covers an outer pheriphry of the sleeve 82; and a seal 84 that has a rubber seal lip for sealing an annular space formed between the cover 83 and the sleeve 82. In this torque transmission device, the shaft 81 and the sleeve 82 are incapable of relative rotation in a circumferential direction, but capable of relative movement in an axial direction, by means of the male spline 81a and the female spline 82a. 
When, specifically, the torque transmission device is used as a joint of a propeller shaft, an incoming obstacle from road (such as a small stone) may hit the seal 84, and therefore it may cause the breakage of the seal 84. For this reason, a structure as shown in FIG. 7 has been proposed such that a second cover 85 that has a cylindrical shape and that coves the seal 84 as well as the cover (first cover) 83 from radially outside is attached to the sleeve 82.
According to the structure of FIG. 7, although the related art can protect the seal 84 from hitting by the incoming obstacle from the outside, the obstacle such as mud and small dirt may get in the inside where the seal 84 is provided through a gap between an inner peripheral surface of the second cover 85 and an outer peripheral surface of the first cover 83. In this case, although the seal 84 with the rubber seal lip prevents the obstacle from entering in a spline portion 86, the obstacle may accumulate in the vicinity of the seal 84, and the accumulated obstacles may eventually cause the breakage and deterioration of the seal 84.
Therefore, a structure is conceived such that, besides the seal 84 (first seal), the second seal having the same rubber seal lip as the first seal 84 is provided between the outer peripheral surface of the first cover 83 and the inner peripheral surface of the second cover. However, in the structure that is provided with the seal, when the shaft 81 and the sleeve 82 relatively move in the axial direction, the volume of the sealed space that is formed between the first seal 84 and the second seal may vary, and the space may be subjected to negative pressure. In this case, due to a pumping action based on the negative pressure, lubricant in the spline portion 86 may be drawn to the annular space between the sleeve 82 and the first cover 83 or sucked outside over the seal 84. Thus, lubricant shortage in the spline portion 86 may occur.